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        function Foo(){

        }
         
        var  foo = new Foo();
        /* 
          访问实例对象的constructor属性本质上是在找.__proto__.constructor
          因为只有原型对象才有constructor属性
          而原型对象的constructor属性,是找哪个构造函数的内部定义了 哪个函数.proptype = {}
        */
        console.log(foo.constructor);
        console.log(Foo.prototype.constructor);
       




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